Cobalt-containing iron oxide magnetic particles have a higher coercive force than that of the conventional iron oxide magnetic particles which do not contain cobalt, and have various advantages in that when they are used as a recording element for a magnetic recording medium, a high density recording can be achieved.
Various methods for preparation of the cobalt-containing iron oxide magnetic particles have been reported. For instance, a useful method is disclosed in Japanese Patent Publication No. 49475/1974. According to this method, the cobalt-containing iron oxide magnetic particles are prepared by dispersing iron oxide magnetic particles having a spinel structure, i.e. a ferrimagnetic .gamma.-ferric oxide (.gamma.-Fe.sub.2 O.sub.3) or ferrosoferric oxide (Fe.sub.3 O.sub.4), into a mixture of an aqueous solution containing an iron salt and a cobalt salt and an alkaline aqueous solution having a concentration of an alkali wherein at least cobalt hydroxide is precipitated, and introducing an oxidizing gas into the dispersion to cause an oxidation reaction to proceed in the dispersion, thereby epitaxially crystallizing cobalt-containing ferrite on the surface of the iron oxide magnetic particles. This method can give acicular cobalt-containing iron oxide magnetic particles having a high coercive force and a uniform particle size and having an excellent heat stability, e.g. excellent print through effect and less demagnetization due to heat.
However, according to the above method, since an oxidation reaction by an oxidizing gas proceeds during the epitaxial growth of the cobalt-containing iron oxide on the iron oxide magnetic particles, the layer of the cobalt-containing iron oxide formed on the iron oxide magnetic particles contains little or no divalent iron, and hence, even if Fe.sub.3 O.sub.4 containing a large amount of a divalent iron is used, the obtained magnetic particles have some defects. For instance, when the magnetic particles thus obtained are used as a recording element for a magnetic recording medium (e.g. a magnetic tape), the obtained magnetic recording medium unfavorably has a comparatively large surface electrical resistance, and hence, easily charges with electricity. In order to eliminate the defects, a conductive material such as carbon black is added to the magnetic composition, but an increase of non-magnetic components unfavorably effects on the characteristics of the magnetic recording medium, for instance, decrease of the sensitivity thereof.